I find it a little funny that we're speculating and extrapolating on how closely playing fast is related to being a bad songwriter, when the topic itself is about probably the absolute worst example of that thing. I don't think it's necessarily indicative of a general trend that much.
I'm not a huge instrumental music fan, I like vocals with mine, so I'm not that deeply into the shredders in general. But I do like some songs from some of the big names, so here's some examples of what I consider to be good songs (or if you don't like them, at least it should be obvious it's not just shredding for the sake of shredding) by people who can play really fucking fast:
(technically a Racer X song and not solo work - but just fucking look at that pinky finger, it's a little unfair. Scarified would also be a great pick)
And a little less known bonus:
Even when they go fast, it has a purpose. Of course there's also often a
lot of flashy and fast playing, but I think that partially comes with having an instrumental song - when you're carrying the whole song with your guitar, you
have to do a lot of interesting things to keep it going, and that also means resorting a lot to playing fast, IMO. I guess jazz people also do instrumental songs with slow playing, so it's absolutely possible, but I don't really like jazz so I can't talk about that.